1. Field of the Invention
This invention relates generally to multiprocessing digital computer systems and particularly to instruction set architecture support for passing messages between concurrently executing processes and for synchronizing concurrently executing processes.
2. Background Information
The application of Merwin H. Alferness, et. al., Docket Number RA-3317, discloses a system architecture for providing improved message passing and process synchronization capabilities. Critical enhancements are made to operating system functions to permit processes executing within this system architecture to pass large messages between them without incurring large performance penalties associated with multiple copy operations on the message data. High level language support is provided in this system to enable an applications programmer to easily use the added functionality of the system. Hardware instruction set architecture support is also provided by the system to ensure that transfers of data and synchronization of communicating processes take place at machine speeds, rather than through multiple software layers of process control in the operating system.
The system architecture, known as the "Queuing Architecture," uses queues as a mechanism for message passing and process synchronization. A queue client process places entries or events on a queue. A queue server process receives entries or events from a queue. An entry contains a message passed between a client process and a server process over the queue. The message consists of data or control information. An event is an indication that a condition known to both the client process and server process has occurred, but which contains no message. Thus, an event works as a synchronization mechanism between processes. Each entry on a queue is represented in a unit of storage called a queue bank. The queue bank has a control area and a text area. The control area contains control information and pointers to other entries on a queue. The text area contains the message data. In the preferred embodiment of the present invention, the text area of a queue bank is limited in size to 262,144 36-bit words. A queue bank may be a queue header or a queue entry. A queue is made up of one queue header and zero or more queue entries. The queue header holds control information for the queue. Queue entries hold the message data being passed between processes. To pass a message from one process to another process in the Queuing Architecture, the sending process inserts the message data into a queue entry and then enqueues it to a queue. The receiving process, which may be waiting on entries being placed on the queue, dequeues the queue entry and princesses the message data.
The implementation of the dequeue operation is of critical importance tier this system architecture. If the act of removing a queue entry from a queue is too slow, overall performance of the system suffers because of the frequency of use of the dequeue operation. Furthermore, the dequeue operation should be performed without copying the message data contained in the queue entry in order to maximize system throughput. Existing message passing systems usually copy the message data from the sending process's virtual space into the system memory used to represent a mailbox. The data is then copied from the mailbox into the receiving process's virtual space. Ideally, the processing time required to perform this copying of the message data should be eliminated. It would be more efficient if a mechanism was provided in the architecture of the system such that the message data could be transferred between processes via a shared queue structure without the need for copying. If the dequeue operation could be performed by the hardware of the system, rather than by the operating system kernel, system performance could be greatly improved.